1. Field of the Invention
This invention relates to an automatic/manual adjuster for a drum brake.
2. Description of the Prior Art
In an automatic adjuster of this type used for a conventional wedge brake as shown in FIG. 1 of the accompanying drawings, an adjuster nut 5 is concentrically disposed within a piston 4 which is caused to non-rotatively move in the axial direction within a cylinder 2 by the operation of a wedge mechanism with the adjuster nut 5 being arranged to be rotatable and to be movable together with the piston 4. This adjuster nut 5 has an adjuster screw 7 attached thereto by screwing. Further, there is provided a lever 9 which is pivotally connected to a body 3 of the cylinder 2. A claw part 10 which is formed at the fore end of the lever 9 is caused by the spring force of a spring 11 to resiliently engage ratchet teeth formed on the outer circumference of the adjuster nut 5.
The operation of the conventional automatic adjuster is such that, during brake application, the adjuster nut 5 and the adjuster screw 7 which is screwed to the adjuster nut 5 move forward together as the piston 4 moves forward in the direction of an arrow shown in FIG. 1. Accordingly, a brake shoe 8 comes into pressed contact with a drum, which is not shown, and there develops a braking force. Then, the forward movement of the adjuster nut 5 which takes place as the piston 4 moves forward sways the claw part 10 of the lever 9 which is in resilient engagement with one of ratchet teeth 6. The degree of the sway of the claw part 10 gradually increases as the degree of the wear of the lining of the brake shoe 8 increases. The engaging end of the claw part 10 comes to deflect to a greater extent in the circumferential direction of the ratchet teeth 6 according as its swaying extent increases and, if it skips one tooth, it engages with the next tooth. When the brake is released under this condition, the brake shoe 8 is urged by the spring force of a return spring, which is not shown, to return to its initial position while pushing the piston 4. However, the return swing of the lever 9 then causes the adjuster nut 5 to rotate by one tooth.
With the adjuster nut 5 thus rotated in the direction of b as shown in FIG. 1, the nut 5 is screwed outward. The adjuster nut 5 and the piston 4 are thus arranged always to return to their initial positions. However, the returning positions at which the adjuster screw 7 and the brake shoe 8 are redetermined are closer toward the drum as much as the relative displacement in the axial direction caused by the rotation of the adjuster nut 5. In this manner, an automatic adjusting action is performed by the outward screwing rotation of the adjuster nut 5 which takes place according as the wear of the lining of the brake shoe 8 increases.
The conventional automatic adjuster which is capable of performing an automatic adjusting operation presents a problem that it virtually prohibits manual adjustment of a clearance between the brake shoe and the drum as desired. To make the manual adjustment, the adjuster screw 7 is rotated to screw it outward in the direction a as shown in FIG. 1 to engage the brake shoe 8 with the drum. Following that, the screw 7 is rotated in the direction of b to a certain given angle and is thus returned to a predetermined extent to have a prescribed clearance between the brake shoe and the drum. However, during the process of returning the screw 7, the frictional force of the screw part causes the nut 5 also to rotate in the same direction of b to hinder the manual adjustment work. This is because the lever 9 does not function to prohibit the nut 5 from rotating in the direction b.